(1) Technical Field
Various embodiments described herein relate to phase shifters and attenuators and more particularly to variable phase shifters and attenuators having self-matching impedance.
(2) Background
From time to time, it is desirable to be able to variably alter the phase of an electrical signal within an electrical circuit. A phase shift is a movement along the time axis of a signal in which voltage or current is plotted against time. FIG. 1 shows a solid line 101 representing three periods of a first sine wave plotted with voltage in the vertical axis and time in the horizontal axis. A dotted line 103 represents three periods of a second sine wave shifted by 90 degrees with respect to the first sine wave 101. It can be seen that the second sine wave 103 is a perfect replica of the first sine wave 101 with the exception of the phase shift (i.e., the sliding along the time axis). Accordingly, it can be said that the sine wave 101 was shifted 90 degrees without any distortion.
A variable phase shifter is a circuit in which a signal coupled to an input port of the phase shifter is shifted in phase and coupled to an output port. FIG. 2 is an illustration of one phase shifter 200 in which the amount of the phase shift can be varied. The variable phase shifter 200 uses several incremental phase shifter elements 202 coupled in series. Each is capable of either providing an incremental phase shift. Alternatively, each of the incremental phase shifter elements 202 can be bypassed (i.e., the signal shunted around that particular incremental phase shifter element 202). In some cases, it is desirable for the variable phase shifter 202 to be able to select the amount of phase shift in very fine discrete steps, such as steps of 0.25 degrees.
FIG. 3a and FIG. 3b illustrate two ways in which the incremental phase shifter elements 202 of FIG. 2 can be implemented. In the case of the phase shifter 202a shown in FIG. 3a, an impedance device 301, such as a capacitor (as shown in FIG. 3a) or an inductor (not shown for the sake of simplicity) having an reactance Z1 is connected to a switch M1. One terminal of the impedance device 301 is coupled to a conductor coupled between an RF (radio frequency) input port 305 and an RF output port 307 of the incremental phase shifter element 202a. The other terminal of the impedance device 301 is coupled to a first terminal of the switch M1. The second terminal of the switch M1 is coupled to ground. Accordingly, when the switch M1 is closed, the impedance device 301 shunts the signal to ground. Adding the impedance device 301 will cause a phase shift, the magnitude of which is determined by both the amount of reactance Z1 and the frequency of the signals applied to the input of the incremental phase shifter element 202a. In some cases, a phase shifter with relatively small discrete step size can be achieved using a capacitive incremental phase shifter element, such as the element 202a shown in FIG. 3a. 
FIG. 3b illustrates an alternative architecture for implementing the incremental phase shifter element 202. The incremental phase shifter element 202b comprises an impedance device 301 having a reactance Z1, such as an inductor having an inductance Z1 (as shown in FIG. 3b) or a capacitor having a capacitance Z1 (not shown for the sake of simplicity) coupled between an RF input port and an RF output port. A switch M1 is coupled in parallel with the impedance device 301. When the switch M1 is open, the signal passes through the impedance device 301. The impedance device 301 causes a phase shift. The magnitude of the shift is determined by the amount of reactance Z1. Alternatively, when the switch M1 is closed, the signal flows through the switch M1 and so bypasses the impedance device 301. Accordingly, with M1 closed, there is ideally no phase shift imposed on the signal as it passes through the incremental phase shifter element 202b. 
However, in some instances there is a problem with using a phase shifter such as the phase shifter 200 having elements such as those shown in FIGS. 3a and 3b. Using incremental phase shifter elements 202a, 202b that rely on a reactive device (such as a capacitor or inductor) to induce the phase shift results in a reactive impedance being imposed on the input and output of the phase shifter 202. The amount of the reactive impedance is dependent upon the amount of the shift imposed by the element 202a, 202b. For example, FIG. 4 is an illustration of a schematic of a capacitive variable phase shifter 202a. As the size of the phase shift imposed on the signal increases, the amount of capacitive loading required to achieve the desired phase shift also increases. Therefore, the input and output impedance of the phase shifter will vary depending upon the amount of phase shift being applied to the signal. In some instances, this can be problematic, since it may be important to maintain an input and output impedance that is matched to the components coupled to the input and the output of the phase shifter to prevent reflections, distortion and loss of power as the signal traverses the circuit. The same problem is also present if the phase shifter 200 is designed using the circuit block 202b with several elements 202b connected in series. That is, as the user increases the number of active phase shifter elements 202, the RF lines get longer. This results in more inductive reactance, thereby disrupting the impedance match and increasing reflections on the RF line. The more elements 202b are added, the worse the return loss gets. Therefore, even if the each element 202 is well matched, using several elements 202 in series will negatively affect the overall impedance match and increase the return loss.
Therefore, there is a currently a need for a variable phase shifter that can self correct the impedance to match the impedance of the circuits to which the phase shifter is coupled at its input and output over a range of selectable phase shifts.